docs: def show_current_number(parser, token):
      """Show the current page number, or insert it in the context.
  
      This tag can for example be useful to change the page title according to
      the current page number.
  
      To just show current page number:
  
      .. code-block:: html+django
  
          {% show_current_number %}
  
      If you use multiple paginations in the same page, you can get the page
      number for a specific pagination using the querystring key, e.g.:
  
      .. code-block:: html+django
  
          {% show_current_number using mykey %}
  
      The default page when no querystring is specified is 1. If you changed it
      in the `paginate`_ template tag, you have to call  ``show_current_number``
      according to your choice, e.g.:
  
      .. code-block:: html+django
  
          {% show_current_number starting from page 3 %}
  
      This can be also achieved using a template variable you passed to the
      context, e.g.:
  
      .. code-block:: html+django
  
          {% show_current_number starting from page page_number %}
  
      You can of course mix it all (the order of arguments is important):
  
      .. code-block:: html+django
  
          {% show_current_number starting from page 3 using mykey %}
  
      If you want to insert the current page number in the context, without
      actually displaying it in the template, use the *as* argument, i.e.:
  
      .. code-block:: html+django
  
          {% show_current_number as page_number %}
          {% show_current_number
              starting from page 3 using mykey as page_number %}
  
      """
      # Validate args.
      try:
          tag_name, args = token.contents.split(None, 1)
      except ValueError:
          key = None
          number = None
          tag_name = token.contents[0]
          var_name = None
      else:
          # Use a regexp to catch args.
          match = SHOW_CURRENT_NUMBER_EXPRESSION.match(args)
          if match is None:
              msg = 'Invalid arguments for %r tag' % tag_name
              raise template.TemplateSyntaxError(msg)
          # Retrieve objects.
          groupdict = match.groupdict()
          key = groupdict['key']
          number = groupdict['number']
          var_name = groupdict['var_name']
      # Call the node.
      return ShowCurrentNumberNode(number, key, var_name)